1. Field of the Invention
The present invention relates to chemical mechanical polishing (CMP), and in particular to methods and apparatus for removing an oxide film from a protective film formed over a substrate having depressions or recesses by chemical mechanical polishing.
2. Background Information
In recent years, in the manufacture of semiconductor devices, mounting density has been increasing and element construction has been rapidly becoming finer. Design rules are already shifting to the order of sub-half micrometers (i.e., submicrons). To realize such fine dimensions, CMP techniques in the planar process are employed for the surface of the substrate material. This improves the flatness (planar property) of the pattern exposure surface in the manufacture of semiconductor devices. As a result, it is possible to improve yield and device reliability. CMP has therefore become an indispensable technique in the planar process of interlayer insulation films formed on the substrate material surface, and in other processes such as separating elements.
For separation of elements that are larger than 0.5 micrometers, a technique known as Local Oxidation of Silicon (LOCOS) has been used. For elements that are finer than 0.5 micrometers, Shallow Trench Isolation (STI) methods are used to cope with the decrease in the element separation width. To use STI, CMP is first employed to remove excess oxide film, such as silicon dioxide laid on a substrate. As a preparatory layer for an oxide film, a protective film of silicon nitride (SixNy) is typically formed as a stop layer for the polishing.
While silica-based slurries have been widely used as a polishing agent in CMP processes, cerium-based slurries containing cerium particles are increasingly being used in the manufacture of semiconductor devices with submicron elements. This is due to the high selectivity between the oxide film and the protective film (high polishing speed ratio) as well as other factors such as improved polishing speed, lower pattern dependence of the polished surface, reduction in metallic impurities, etc.
In a CMP process, the excess portion of the oxide film must be removed appropriately, and therefore control over the polished amount is important. The detection of a stop or end point of the polishing is also important. To accurately detect the polishing end point, various methods are used. Japanese Patent Publication No. 3177549, Patent Publication No. 6-31850, and Patent Publication No. 10-202522 describe methods or devices for detecting the end point of a CMP process by indirectly measuring the polishing amount from the torque of the substrate holder during polishing, its differential value, and the change in its integral values. A more accurate method for detecting the end point is also disclosed using a laser beam to measure the thickness of the surface being polished and determine the polished amount. The latter method is generally deemed to be advantageous for situations where a higher accuracy is required for the end point detection, such as situations associated with finer processing dimensions.
Unfortunately, conventional optical methods for end-point detection in CMP processes using cerium-based slurries have shown some difficulty in reliably detecting the end point. The optical detection intensity is poor when compared to silica-based slurries used under the same optical measurement conditions, and satisfactory improvement was not recognized. It is believed that light scattering through the cerium-based slurry may cause these problems.
End point detection by the conventional torque methods has been attempted using cerium-based slurries. Both real-time torque values and differential/integral values of the change in torque have been used for end-point detection. In both cases, accurate detection of the end point was prone to be difficult. In many cases, the end point was either not detected or a plurality of end points were detected.
Accordingly, the use of CMP with cerium-based slurries, particularly when forming elements by STI, presents process management or control problems. Methods and devices as therefore needed that allow for adequate process management and process control when using these CMP techniques.